The discovery of the laser in 1960 spurred promises of transmitting information over beams of light. But it was soon evident that a practical waveguide medium for transmitting light signals over significant distances did not exist. However, the recent development of optical fibers through which optical information can be transmitted with remarkably low loss has made optical communication appear viable.
Optical fibers for transmitting light signals have been proposed in many forms. An especially promising fiber is the so-called "clad fiber". This fiber relies on total internal reflection at the interface between a fiber core and a surrounding cladding to contain the light within the core of the fiber. The core is made of a highly pure glass material in which light absorption and scattering losses are minimal. The cladding material has an index of refraction that is less than that of the core.
From a material standpoint, many optical fiber structures rely on some form of fused silica. Silica is a chemically stable, simple material and can be made in extremely pure form. Since the mechanism which causes absorption losses in very low loss fibers involves chemical impurities (particularly transition metal ions and water) the purity of the material is important.
Fused silica has one of the lowest indices of refraction of all glasses and therefore it cannot easily serve as the core in a clad glass fiber. An alternative is to modify highly pure silica chemically to increase its refractive index enough that the modified material becomes the core material and the fused silica the cladding material. This is one of the "clad fiber" structures that has gathered attention and favor.
Clad fiber structures are conventionally made by chemical deposition processes. For a description of exemplary techniques see U.S. Pat. No. 3,659,915 issued to R. D. Maurer and P. C. Schultz; U.S. Pat. No. 3,711,262 issued to D. B. Keck and P. C. Schultz; Applied Physics Letters, 23 (6) 338-339 (1973) by W. G. French, A. D. Pearson, G. William Tasker and J. B. MacChesney entitled "Low Loss Fused Silica Optical Waveguide with Borosilicate Cladding"; and Applied Physics Letters, 23 (6) 340-341 (1973) by J. B. MacChesney, R. E. Jaeger, D. A. Pinnow, F. W. Ostermayer, T. C. Rich and L. G. Van Uitert entitled "Low Loss Silica Core-Borosilicate Clad Fiber Optical Waveguide."
In the well-known modified chemical vapor deposition (MCVD) process for forming optical waveguides, volatile metal halides (such as SiCl.sub.4, GeCl.sub.4, and BCl.sub.3) are entrained in an O.sub.2 stream which is passed through a rotating fused silica support tube. The tube is heated by a motor driven H.sub.2 /O.sub.2 torch to temperature sufficient to initiate the MCVD reaction in the hot zone. This results in the deposition of a clear glassy layer. After the deposition of such layers is complete, the tube is further heated, causing it to collapse and form a solid rod preform. The refractive index profile of the preform can be controlled by varying the ratio of the various halides used. Fibers drawn from such preforms exhibit very low loss and their compositional (and thus refractive index) profiles can be tailored precisely.